Recent developments have enabled wireless local area networks (WLANs) to be deployed for outdoor uses. However, in outdoor scenarios, wireless channels in a WLAN are often affected by the Doppler effect, for example caused by movements of associated wireless stations (STA) or fast-moving objects around the STAs. As a result, the wireless channels may vary with time during transmission of a packet. If such channel variation is not taken into account in transmitting or receiving the packet, transmission performance in the WLAN system tends to be adversely affected.
An existing approach to solve the temporal channel variation issue involves inserting a “mid-amble” in the data field of a packet, the mid-amble including training symbols used by a receiving device to perform channel estimation and thereby keep track of the channel condition in real time. The mid-amble may be a repeat of a high efficiency-short training field (HE-STF) or a high efficiency-long training field (HE-LTF) included in the preamble. Typically, a mid-amble is inserted following a predefined data length, e.g., after each predefined transmission time or each predefined number of orthogonal frequency-division multiplexing (OFDM) symbols. Thus, a mid-amble is only applicable if the packet data exceeds the predefined length.
Depending on the implementation of the transmitting device, inserting a mid-amble is optional even for packets having a data length that is greater than the predefined length. So it is necessary to inform the receiving device of the presence of mid-ambles. To this end, the packet preamble uses a dedicated field as an indicator. For example, as in the IEEE 802.11 family of Standard and Specifications, a one-bit “Doppler mode” field in the preamble HE-SIG A field is defined to indicate whether any Doppler mode mid-amble is included in the packet. However, as a mid-amble is only eligible if the packet exceeds a certain length, the “Doppler mode” field cannot provide any meaningful indication for shorter packets and therefore constitutes a waste of an indication resource in the preamble. Thus, for these shorter packets, there lacks a mechanism for a receiving device to acquire knowledge of the current channel condition from a received packet. Unfortunately, constrained by the existent physical layer conformance procedure (PLCP) protocol data unit (PPDU) structures with all the preamble bits already used or reserved for specific indications, it is difficult to introduce any additional field for reporting the channel condition in a shorter packet.